The hitherto-known process for producing a 5-hydroxy-3-oxopentanoic acid derivative includes the following processes.                (1) The process in which 3-hydroxypropionic acid imidazolide prepared from 3-hydroxypropionic acid and diimidazoyl ketone is coupled to a malonic acid monoester monomagnesium salt (Synthesis, 1992, 4, 403-408).        (2) The process in which a lithium enolate prepared from tert-butyl acetate and lithium diisopropylamide is reacted with a 3-hydroxypropionic acid ester (Japanese Kokai Publication Hei-8-198832, Chem. Pharm. Bull., 1994, 42 (11), 2403-2405, Tetrahedron Lett., 1993, 49(10), 1997-2010, Tetrahedron, 1990, 46 (29), 7283-7288, Tetrahedron Asymmetry, 1990, 1 (5), 307-310, Tetrahedron Lett., 1989, 30 (38), 5115-5118, Tetrahedron Lett., 1987, 28 (13), 1385-1388, Synthesis, 1985, (1), 45-48).        
However, the prior art (1) requires an expensive starting material while the prior art (2) involves a very low reaction temperature of −78° C. to −40° C., so that neither is a favorable process for commercial-scale production.